How to Measure for Replacement Windows: Step by Step
Before you start, you need a tape measure (at least 16 feet), a notepad or phone to record measurements, a level (2 or 4 foot), and a screwdriver or awl for checking frame condition. Measure from inside the house with the window closed. If the window has interior stops (vertical strips that hold the sash in place), you will measure to those stops for insert replacement or to the jambs themselves for full-frame replacement.
Step 1: Measure the Width
Open the window or remove the lower sash if possible to access the full opening. Measure the horizontal distance between the left and right jambs (the vertical side pieces of the frame) at three points: across the top of the opening, across the middle, and across the bottom near the sill.
Measure from jamb face to jamb face, not from stop to stop and not from trim to trim. The jamb is the actual structural frame of the window. The stops are the thin strips that hold the sash in the track, and the trim (casing) is the decorative molding around the window on the interior wall. Using stops or trim as reference points will give you an incorrect measurement.
Record all three width measurements and use the smallest one as your order width. Window openings are rarely perfectly rectangular because wood frames shift, settle, and warp over time. Using the smallest measurement ensures the new window will fit the narrowest point of the opening. The installer will shim the remaining gaps during installation.
For standard double-hung windows, the most common widths are 24, 28, 30, 32, 36, and 40 inches. If your measurement falls within 1/8 inch of a standard size, the opening was built for that size and you can order accordingly. If it is between standard sizes, order based on your actual measurement and the manufacturer will build to that dimension.
Step 2: Measure the Height
Measure the vertical distance from the top of the sill (the horizontal bottom piece of the frame, not the stool, which is the interior shelf) to the underside of the head jamb (the horizontal top piece). Take this measurement at three points: the left side, the center, and the right side.
The sill is the sloped piece that directs water outward. Measure from the highest point of the sill's interior surface, not from the stool or the window apron below it. The head jamb is the top of the frame opening. Measure to the jamb itself, not to the head casing or any stops.
Record all three height measurements and use the smallest one as your order height. Common standard heights are 36, 44, 48, 52, 54, and 60 inches. The same rule applies, if your measurement is within 1/8 inch of a standard size, the opening was framed for that size.
A common mistake is measuring to the wrong reference point. The stool (the flat interior shelf where people put plants and picture frames) extends past the sill on both sides and sticks out from the wall into the room. It is not part of the window frame. Measuring from the stool gives you a height that is too short, resulting in a window that leaves a visible gap at the bottom.
Step 3: Measure the Depth
Measure the depth (also called the jamb depth or frame depth) from the interior face of the frame to the exterior face. This tells you how thick the frame cavity is and determines whether a standard-depth replacement window will fit or whether you need a specific jamb extension.
Most replacement windows require a minimum frame depth of 3.25 inches. Standard residential window frames are 3.25 to 3.5 inches deep in homes built after 1960. Older homes with thicker walls may have frame depths of 4 to 5 inches, which is fine because the extra depth simply means the new window will sit deeper in the opening. Newer homes with 2x6 wall framing have frame depths of 5.5 to 6.5 inches and may need jamb extensions (additional frame pieces that fill the extra depth).
If your frame depth is less than 3.25 inches, which is uncommon but possible in some mobile homes, additions, or non-standard construction, you will need a narrow-jamb replacement window or a custom order. Alert your contractor or supplier about the shallow depth before ordering.
Step 4: Check for Square
Measure diagonally from the top-left corner to the bottom-right corner of the opening, then from the top-right corner to the bottom-left corner. In a perfectly square opening, both diagonal measurements are identical. In reality, some variation is normal.
If the two diagonals are within 1/4 inch of each other, the opening is square enough for a standard insert replacement window. The installer will use shims to correct minor discrepancies during installation.
If the diagonals differ by more than 1/4 inch but less than 1/2 inch, the opening is moderately out of square. A skilled installer can still work with this using strategic shimming, but the new window may show a slightly uneven reveal (the visible gap between the window frame and the wall opening). Discuss this with your contractor before ordering.
If the diagonals differ by more than 1/2 inch, the opening is significantly out of square. This usually means the frame has shifted due to foundation settling, structural movement, or water damage. An out-of-square opening greater than 1/2 inch typically requires full-frame replacement so the installer can rebuild the opening square, or the window must be custom-manufactured to the out-of-square dimensions, which adds 20% to 40% to the cost.
Step 5: Inspect the Sill and Frame
Before finalizing your measurements, inspect the condition of the existing frame because it determines whether you can do an insert replacement or need full-frame installation.
Press a screwdriver or awl into the sill at several points, especially the corners and any areas where paint is peeling, cracking, or bubbling. Do the same on the lower portions of both side jambs. Sound wood feels hard and resists the probe. Soft, spongy, or crumbly wood indicates rot that has compromised the frame's structural integrity.
Check the sill for slope. The sill should slope slightly toward the exterior (about 1/8 inch per inch of depth) to drain water outward. A flat or reverse-sloped sill traps water and accelerates rot. If the sill has reversed its slope due to settling, it needs to be corrected during replacement.
Look at the exterior brick mold or trim around the window. Gaps between the trim and the siding, cracked caulk, or separating joints indicate that water may have been entering the wall cavity around the window. Document what you find so you can discuss it with the installer.
Step 6: Record and Order
Create a record for each window that includes the smallest width, smallest height, frame depth, squareness check result (within 1/4 inch or not), frame condition (sound, surface rot, deep rot), and the room location. Label each window so you and the installer are talking about the same opening when reviewing the order.
When ordering, provide the smallest width and smallest height. The manufacturer builds the replacement window slightly undersized, typically 1/4 to 3/8 inch smaller in each dimension, to allow room for shimming, insulation, and a proper fit. Do not deduct this tolerance yourself. If your smallest width measurement is 29-3/4 inches, order 29-3/4 inches, and the manufacturer will produce the window at approximately 29-3/8 to 29-1/2 inches.
Double-check every measurement before ordering. Measure each opening at least twice on separate occasions to catch errors. A window manufactured to wrong dimensions cannot be returned for a refund in most cases because it was custom-built to your specifications. This is especially critical for non-standard sizes where a reorder means waiting another 3 to 6 weeks for manufacturing.
Common Measuring Mistakes to Avoid
Measuring trim to trim instead of jamb to jamb. The interior casing (trim) sits on top of the wall surface and extends past the actual frame opening. Measuring from trim to trim gives you a number that is 1 to 3 inches too large in each direction.
Measuring from the stool instead of the sill. The stool is the flat shelf, the sill is the sloped piece beneath it. The sill is your reference point for height measurements. Measuring from the stool underestimates the height by 3/4 to 1.5 inches.
Taking only one measurement per dimension. A single measurement assumes the opening is perfectly rectangular, which it almost never is. Always take three measurements across the width and three across the height, then use the smallest of each.
Rounding measurements. Window sizing is precise to 1/8 inch. Rounding 29-5/8 to 30 inches will result in a window that does not fit. Record exact fractions.
Forgetting to measure the depth. A window that is too deep for the frame protrudes past the interior wall surface and cannot be properly trimmed. A window that is too shallow leaves a gap that must be filled with a jamb extension.
When to Let the Contractor Measure
Most window companies and contractors include free measurement as part of the estimate process. If you are ordering through a contractor or a manufacturer's installer, let them do the final measurements because they carry the responsibility for accuracy. Your own measurements are still valuable as a cross-check and for getting preliminary quotes before committing to a contractor.
DIY measurement is essential if you are ordering windows yourself and hiring a separate installer, or if you are doing the installation yourself. In these cases, you bear full responsibility for measurement accuracy, so measure each window at least twice and have someone verify your work before placing the order.
Always measure width and height at three points and use the smallest reading. Measure jamb to jamb, not trim to trim. Check for square with diagonal measurements. Inspect the frame for rot before deciding on insert or full-frame replacement. Record exact fractions, never round, and measure every opening twice before ordering.